The present invention relates to quality of recording in information storage devices such as disk drives, and in particular, to measuring the surface temperature of the storage medium, typically a disk, for high areal density recording.
With increasing demand for data storage there is a corresponding demand for increasing recording density on storage media such as magnetic disk drives. Heads with an integrated inductive writer and a magneto-resistive (MR) reader are utilized in many disk drives for recording and reading data in concentric tracks on a data disk coated with a magnetic medium. The recording density on such disk is a function of the bits-per-inch (BPI) and tracks-per-inch (TPI) density of the recorded data.
As the recording density is increased the size of the magnetic grains in the medium is reduced. The smaller grains can become thermally unstable, wherein the magnetization direction spontaneously reverses due to thermal energy at finite temperature. In that process, the stored information is lost. Attempting to prevent this effect by increasing the medium anisotropy and coercivity, which improves the thermal stability of the magnetic grains, is problematic because it is difficult to generate sufficient magnetic field with the writer to record on such media.
Hybrid recording represents a potential solution to this problem. Hybrid recording uses a laser and magnetic field to write, and a magneto-resistive read transducer with or without a laser to read. In the former case, the laser elevates the medium temperature, thereby lowering the medium anisotropy and coercivity sufficiently such that the write element can reverse the medium magnetization and record information. As the medium cools, the anisotropy and coercivity increase, thereby improving the thermal stability in long-term storage.
Certain versions of Hybrid recording also use a laser in the read process. The laser is used to increase the temperature of the magnetic material in the medium. This material can be a ferri-magnet, which has at least two magnetic sub-lattices arranged such that the net magnetization is zero at a particular temperature. Changing the temperature increases the magnetization of a readout layer, such that a particular track can be read back (neighboring tracks are at lower temperatures and thus the read back signal is significantly smaller). Current Magneto-Optic recording uses the thermal profile in the storage medium to define the location of reversals in the medium magnetization direction (transitions), and the track width (in the write and/or read process). Hybrid recording can use the thermal profile, the magnetic field profile (of the fields produced by the recording head), or both, to define the transition locations and track width.
The thermal profile in the medium is a function of the thermal properties of the medium, the laser spot size and shape, the laser power and the ambient drive temperature. For a given medium and optical system, the effective write width is a sensitive function of the thermal profile in the cross-track direction. In that case, changes in the peak temperature in the medium leads to significant changes in the write width and, therefore, the track density.
To achieve a high track density (small write and read widths) in a Hybrid recording process, it is important to determine and control the thermal profile in the medium. Possible techniques include using the ratio of the Stokes/Anti-stokes intensity of a particular Raman mode associated with the overcoat or optical pyrometry, where the disk surface temperature is determined by infra-red radiation absorbed by a detector. However, such techniques are complex and require additional optical components, which make them expensive and difficult to implement in a practical storage device.
There is, therefore, a need for a simple method for determining the medium temperature in Hybrid recording to enable high track density. Furthermore, there is also a need for such a method to allow compensation for changes in the ambient temperature and variations in the laser power over its life. There is also a need for such a temperature measuring method to enable laser power shaping of the spot size/thermal contour in the medium (allowing smaller spot sizes to be defined with a given optical system).
The present invention satisfies these needs. In one embodiment, the present invention provides a technique to determine the recording medium temperature, thereby enabling high track density in Hybrid recording. This allows compensation for changes in the ambient temperature and variations in the laser power over its life. Such a temperature measuring technique also enables laser power shaping of the spot size/thermal contour in the medium, providing smaller spot size definition with a given optical system. Baseline modulation is a sensitive probe of the temperature gradient between a magneto-resistive read element (anisotropic or giant) and the surface of a disk. By calibrating the temperature rise (above ambient) of the read element, this technique can be used to determine the disk surface temperature. This information is useful in high areal density Hybrid or conventional Magneto-Optic recording applications.